4 SYMBIOSIS 



mutual effort the economic problem of existence. Systematic 

 economic co-operation in this case of " attached " Symbiosis 

 has led to a high degree of reciprocal adaptation and 

 reciprocal differentiation in the physiology of the organisms 

 concerned. Indeed the physiological reciprocity has here become 

 so intimate that it required years of painstaking research to 

 establish the fact of the compound and dual nature of the 

 lichen. 



It is generally a higher fungus which is thus found to be 

 associated with a generally unicellular, sometimes filamentous 

 alga. The special fungi which take part in the association are, 

 with rare exceptions, not found growing separately, whilst the 

 algal forms are constantly found free. The algal forms thus 

 have retained their primitive independence rather more than 

 the fungus, which latter, on the other hand, has stamped its 

 character more prominently upon the compound inasmuch as 

 the reproductive organs of the lichen are of a typically fungal 

 character. The algal cells are never known to form spores whilst 

 forming part of the lichen-thallus, but they may do so when 

 separated from it and growing free. "The fungus," says the 

 Encyclopedia Britannica, " clearly takes the upper part in the 

 association." 



The fungus, in virtue of its bio-chemical equipment, is better 

 qualified than the alga for the labours of sex. But though the 

 alga, by restraining its own reproductive tendencies, as the 

 Encyclopaedia says, plays a subordinate part, the part played 

 by the alga is of considerable and far-reaching importance. For 

 the better the associated fungus specialises as regards the 

 reproductive function, more exclusively deputed to it, the 

 better the alga is able to perform its own special photosynthetic 

 duties," i.e., to manufacture essential food and even a surplus 

 of such food, and often various other valuable substances domes- 

 tically and bio-economically important, which are stored up in 

 the compound organism as capital to facilitate further develop- 

 ment. 



Lichens are able to live in situations where neither the alga 

 nor fungus could exist alone. The alga is protected by the 

 threads (hyphae) of the fungus, and supplied with water and 

 salts and, possibly, organic nitrogenous substances, and, in turn, 

 it manufactures photosynthetically carbohydrates, the surplus 

 of which it yields to the fungus. This form of relationship is 



